[exporter/stefexporter] Mark STEF exporter as Alpha

### Description

Mark STEF exporter as `Alpha` stability.

### Testing Done

Tested manually with STEF Server here https://github.com/splunk/stef/tree/main/otelcol/cmd/stefmockserver

Built Collector contrib via `make otelcontribcol`. Used the following config for testing:

```yaml
receivers:
  hostmetrics:
    collection_interval: 1s
    scrapers:
      load:
      filesystem:
      memory:
      network:
      paging:
      processes:

exporters:
  debug:
  stef:
    endpoint: localhost:4320
    tls:
      insecure: true

processors:

service:
  pipelines:
    metrics:
      receivers: [hostmetrics]
      processors: []
      exporters: [debug,stef]
```

#### Scenario 1

- Started STEF Server, started Collector, observed data correctly received by Server.
- Stopped Server, restarted Server, observed Collector disconnecting and then correctly reconnecting to the Server and resuming sending.

#### Scenario 2

- Started Collector, observed Collector unable to send data. Started Server, observed Collector correctly connecting and starting to send data.

.chloggen/tigran_stefexporteralpha.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: stefexporter
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Mark STEF exporter as Alpha
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [37759]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

cmd/otelcontribcol/builder-config.yaml

@@ -98,6 +98,7 @@ exporters:
   - gomod: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/sentryexporter v0.120.0
   - gomod: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/signalfxexporter v0.120.0
   - gomod: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/splunkhecexporter v0.120.0
+  - gomod: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/stefexporter v0.120.0
   - gomod: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/sumologicexporter v0.120.0
   - gomod: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/syslogexporter v0.120.0
   - gomod: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/tencentcloudlogserviceexporter v0.120.0

exporter/stefexporter/README.md

@@ -3,12 +3,13 @@
 <!-- status autogenerated section -->
 | Status        |           |
 | ------------- |-----------|
-| Stability     | [development]: metrics   |
-| Distributions | [] |
+| Stability     | [alpha]: metrics   |
+| Distributions | [contrib] |
 | Issues        | [![Open issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector-contrib?query=is%3Aissue%20is%3Aopen%20label%3Aexporter%2Fstef%20&label=open&color=orange&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector-contrib/issues?q=is%3Aopen+is%3Aissue+label%3Aexporter%2Fstef) [![Closed issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector-contrib?query=is%3Aissue%20is%3Aclosed%20label%3Aexporter%2Fstef%20&label=closed&color=blue&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector-contrib/issues?q=is%3Aclosed+is%3Aissue+label%3Aexporter%2Fstef) |
 | [Code Owners](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/CONTRIBUTING.md#becoming-a-code-owner)    | [@tigrannajaryan](https://www.github.com/tigrannajaryan), [@dmitryax](https://www.github.com/dmitryax) |
 
-[development]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#development
+[alpha]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#alpha
+[contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
 <!-- end autogenerated section -->
 
 Export data via gRPC using

exporter/stefexporter/metadata.yaml

@@ -3,8 +3,8 @@ type: stef
 status:
   class: exporter
   stability:
-    development: [metrics]
-  distributions: []
+    alpha: [metrics]
+  distributions: [contrib]
   codeowners:
     active: [tigrannajaryan, dmitryax]
 tests: